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Rep. John Lewis says Trump is not a 'legitimate' president

Conservative radio host Ben Ferguson says that questioning is unprecedented

Washington (CNN)A war of words is brewing between President-elect Donald Trump and civil rights icon John Lewis after the congressman doubted the President-elect's legitimacy -- and a conservative pundit says the questioning is "unprecedented."

Trump harshly responded to Lewis, tweeting Saturday that he is "all talk" and "no action" after the Georgia Democrat told NBC's Chuck Todd that Trump is not a "legitimate" president.

During an interview on CNN's "Newsroom" with Poppy Harlow on Saturday, conservative radio host Ben Ferguson criticized Lewis for his comments and said he couldn't believe a congressman of Lewis' stature could question the President-elect's legitimacy.

"It is unprecedented," Ferguson said. "I cannot imagine the fallout, the backfire that you would have if a Republican would have ever implied that about Barack Obama or Bill Clinton or JFK, or anyone else for that matter."

Harlow and fellow guest Symone Sanders, the former press secretary for Bernie Sanders' presidential campaign, then began to interrupt Ferguson, reminding him there is a precedent, and it's recent.

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"Ben, that's exactly what many Republicans did," Harlow said, "including the President-elect for years questioning the legitimacy of the first black president."

Trump launched his political career largely by suggesting President Barack Obama wasn't born in the United States and repeatedly calling for him to release his birth certificate.